As examples of a resin material conventionally used for a toner, may be mentioned a polystyrene resin, a styrene-acrylic copolymer resin, a polyester resin, an epoxy resin, a butyral resin, and a hybrid resin such as a polyester resin having a grafted acrylic resin. Such a resin material is designed according to the application of a toner.
In particular, a resin material for a toner that is fixed with a heat roller requires improvement in fixing properties onto a recording medium and offset resistance. Conventionally, a macromolecular weight thermoplastic resin or a partially cross-linked thermoplastic resin has been mainly used.
Many efforts have been made to achieve a toner having excellent low-temperature fixing properties for high-speed and energy-saving printers and copiers. However, when the resin material as described above is used, a temperature at which a toner is fused and fixed (fixing temperature) needs to be set to be high, and it is difficult to save the energy.
In order to obtain a toner having a low fixing temperature, it is necessary to use a substance having a low melting temperature or a low melt viscosity as a resin material. Further, in order to obtain such a resin material, it is important that the resin material has a low glass transition temperature (Tg) or a low molecular weight.
However, such countermeasures additionally cause a problem where a toner has a low heat resistant storage stability (blocking resistance).
Therefore, it is essentially difficult that both the low-temperature fixing properties and the heat resistant storage stability are achieved.
In order to solve such a problem, a toner having a core particle of an amorphous resin and a coating surface of a crystalline polyester resin has been proposed (see Patent Literature 1).
However, the crystalline polyester resin is hard but fragile. In a vinyl-based resin such as a high general-purpose styrene-acrylic copolymer resin, the molecular weight needs to be low to achieve low-temperature fixing properties. In this case, sufficient crush resistance cannot be achieved.
When a toner is used as a two-component developer, the toner is generally mixed with a carrier such as iron powder within a developing device in a development step to generate electrostatic charge due to friction. When a fragile toner among the toners is used for a long time, the toner is broken due to friction with the carrier to produce a finer toner. The fine toner is likely to adhere to the surface of the carrier. Further, fusion of the finer toner to the carrier reduces the electric charge-imparting function of the carrier. Thus, the electric charge amount of the toner is reduced. As a result, the toner that is not charged sufficiently is scattered, to cause a problem of surface fogging on an image.
Patent Literature 2 has proposed a technique of achieving low-temperature fixing properties by mixing an amorphous resin with a crystalline resin with a low melting point to control the degree of compatibility.
However, when the compatibilization of the crystalline resin and the amorphous resin progresses, a mixed resin is plasticized, to cause a problem where sufficient heat resistant storage stability is not achieved.